1. Field of the Invention
This invention relates to polyolefin compositions, and, more particularly, to antioxidant-containing polyolefin compositions which are particularly effective in providing antioxidant stabilization at relatively low concentration levels.
2. Description of the Prior Art
Polyolefins, such as polyethylene, polypropylene and the like, are known to be susceptible to degradation in air. The effect of oxygen upon the polyolefin causes oxidation of carbon-hydrogen bonds in the molecule, resulting in embrittlement, and, color deterioration or yellowing of the polymer. In addition, the polymer can experience a loss of molecular weight upon cleavage in the presence of oxygen, which effect is measured as a change in the melt flow index of the polymer.
Both these oxidative reactions can be slowed down by including an antioxidant material in the polymer composition. Suitable antioxidants for polyolefins are hindered phenols, that is, phenols which are substituted with large alkyl groups in the ortho positions of the phenol. The hindered phenols preferably are used in combination with other antioxidants, including thioesters, e.g. thiodipropionic acid esters; organic phosphites; and hindered amines (HALS). However, such secondary antioxidants require a rather high concentration level to be effective.
Accordingly, it is an object of the present invention to provide new and effective antioxidant-containing polyolefin compositions which will advantageously prevent the polyolefins from undergoing a significant color change in air or of suffering a loss in molecular weight during processing.
Another object of the invention is to provide such compositions in which the antioxidant is effective at relatively low concentration levels.
These and other objects and features of the invention will be made apparent from the following description of the invention.